Mechanical and adhesive advantage of resin bonding agents depend upon the ability to penetrate microspaces created with acid-etchants and close adaptation to cavity surfaces. As a replicating material, these resins may reveal morphologic characteristics of the cavity surfaces not seen by direct scanning electron microscope (SEM) investigation. The purpose of this study was to describe, with the SEM, the resin replications of acid-etched cavity walls of Class V cavity preparations in the labial and lingual surfaces of extracted premolar teeth. Cavities were prepared in the gingival third of these surfaces in 26 freshly extracted human premolar teeth using fissure burs in water-cooled, high-speed handpieces. The cavosurface margins were bevelled. The preparations were washed in tap water, dried, etched for 20 seconds with 35% phosphoric acid, coated with light-cured bonding agent and filled with light-cured composite resin in two applications. The teeth were dissolved in acid and the cavity walls of the composite examined in the SEM. Features observed included: (a) Type II resin penetration of interrod regions, (b) resin penetration of the lamellae to the dentino-enamel junction (DEJ), (c) a 10-20 microns step in surface contour at the DEJ, with penetration of terminal tubule branches, (d) insular regions of deep dentin tubule penetration and (e) 100-300 microns deep, 10-30 microns incremental microlamellar penetrations into the enamel at the DEJ corresponding to enamel tufts.